A spark plug is used in an internal-combustion engine for the purpose of ignition. A general spark plug is comprised of a metallic shell for holding an insulating porcelain in which a center electrode is inserted, and a ground electrode welded to the leading end of the metallic shell. A spark discharge gap is formed between the other end of the ground electrode and the opposite leading end of the center electrode. Spark discharge occurs between the center electrode and the ground electrode.
The metallic shell of the spark plug is fixed to the insulating porcelain by inserting the leading end of the insulating porcelain from its rear end side to the leading end side, and tightening the opening of the rear end side to the insulating porcelain side (inside the radial direction of metallic shell). An annular packing is interposed in the gap between the metallic shell and insulating porcelain. By firmly tightening the insulating porcelain and metallic shell, both sides of the packing are tightly fitted to the insulating porcelain and metallic shell so that air tightness is maintained. Carbon steel, such as SPCC (cold rolled steel) having a hardness nearly the same as that of the metallic shell made of ferrous material, may be used as the material of such packing. Iron or copper, both excellent in heat resistance, may also be used (see, for example, Japanese Patent Application Laid-Open No. Hei 10-73069).
Recently, an increase in the output, and a decrease in fuel consumption are demanded from automotive engines. As a result, a smaller diameter spark plug having a longer reach is requested from the viewpoint of degree of freedom in design of the engine. As the spark plug diameter becomes smaller and the reach, i.e., the length, becomes longer, the wall thickness of the metallic shell is reduced. As a result, the strength of the metallic shell itself is reduced, and it is necessary to reduce the strength of the tightening force. This leads to a reduction of residual stress accumulated in the packing from tightening, and it is difficult to maintain the air tightness. Accordingly, it is proposed to accumulate a larger residual force in the packing by allowing it to be tightened firmly by forming the metallic shell from a material of higher strength.
However, since the metallic shell is usually formed by cutting, i.e., machining, after its forming by forging, if the strength of the metallic shell is increased, forging or cutting is more difficult, and productivity may be lowered.
Japanese Patent Application Laid-Open No. Hei 10-73069 proposes forming the packing by using a material of lower strength than that of the metallic shell. However, unless an appropriate material is selected, the packing may not retain its annular shape when the residual stress generated by tightening is applied thereto, or air tightness may not be maintained, or the packing may not withstand the pressure of tightening and may be broken.
The present invention addresses these problems of the prior art, and provides a spark plug capable of maintaining air tightness by the packing interposed between the insulating porcelain and the metallic shell.